One type of axial fan includes a spider having a central portion for engaging a hub or motive source and a plurality of lobes extending radially outward from the central portion to which fan blades are secured. The lobes of the spider are bent or twisted during manufacture to position the blades, which are attached to the lobes after twisting, at desired pitch angles. In one approach, the spider and fan blades of the axial fan are stamped from sheet metal. An operator loads the spider into a traditional press machine that twists the lobes of the spider to impart desired twist angles to the lobes. The operator removes the spider from the press machine, checks the height of the lobes, rivets the fan blades to the lobes, and verifies that the dimensions and balance of the assembled fan are within acceptable tolerances.
The traditional press machine has a hydraulic ram that clamps the spider between upper and lower forming plates of the machine and causes clamping pads of the forming plates to pivot and twist the lobes of the spider. Cylindrical bearings within each forming plate support the clamping pads and permit the clamping pads to pivot relative to the respective forming plate. Lowering the upper forming plate of the press machine against the lower forming plate causes a linkage to pivot the clamping pads of the lower forming plate which, in turn, causes pivoting of the clamping pads of the upper forming plate. The combined pivoting of the clamping pads of the upper and lower forming plates twists lobes of a spider clamped therebetween and imparts twist angles to the lobes.
One problem with the traditional press machine is that the cylindrical bearings of the forming plates are expensive and wear rapidly when subjected to compressive loads. To extend the lifecycle of the bearings and reduce operating cost, the press machine may be configured so that the upper forming plate does not completely clamp a spider against the lower forming plate. Instead, the spider rests upon the lower forming plate, and the upper forming plate stops at a position approximately ten thousandths of an inch above the spider. Because of the clearance between the upper forming plate and the spider, the lobes of the spider tend to slide circumferentially between the clamping pads as the clamping pads pivot and twist the lobes of the spider. The circumferential sliding of the spider lobes between the clamping pads is a major source of variation in the twist angles of the lobes of each spider and is difficult to control on a spider-by-spider basis throughout a production run.
Another problem with the traditional press machine is that setting up the press machine to produce spiders having desired lobe twist angles is a trial-and-error process which results in a large amount of scrap material. More specifically, at the beginning of a production run, the linkage that controls the pivoting of the clamping pads is adjusted to an initial setting to produce spiders having the desired lobe twist angles. The initial setting is an empirically determined setting that produces lobe twist angles that are substantially similar to the desired lobe twist angles. Next, the operator uses the machine to twist a series of spiders, the operator adjusting the linkage and the pivoting of the clamping pads after every spider until a spider is produced having lobe twist angles within tolerance. Normally, it takes between three and five (or more) spiders in order to setup the press machine and produce a spider having lobe twist angles within tolerance. The spiders produced while bringing the press machine within acceptable tolerances typically have twist angles so far from the desired lobe twist angles that they must be scrapped. Scrapping several spiders in order to setup the press machine makes production runs of only a few spiders cost-prohibitive.
The press machine is considered setup once a spider is produced having lobe twist angles within tolerance. However, subsequent spiders may have lobe twist angles outside of acceptable tolerances such that continued tracking of spider lobe twist angles throughout a production run is needed. The continued tracking is achieved by performing a gauging step on each twisted spider that compares the spider to a spider having the desired lobe twist angles. Using information from the gauging step, the operator uses stepper motors to adjust the linkage of the press machine and the range of pivoting of the clamp pads in an attempt to twist the lobes of the next spider closer to the desired lobe twist angles. In this manner, each spider is used as a data point to adjust the settings of the press machine and bring the lobe twist angles of the subsequent spider closer to acceptable tolerances. Adjusting the press machine using the differences between each twisted spider and the desired lobe twist angles causes the settings of the linkage, the range of pivoting of the clamp pads, and the variation of the spider lobe twist angles, to continually fluctuate as the material properties of the spiders vary throughout a production run.
Another shortcoming of the traditional press machine is that corrections to the spider lobes are done by hand. In one approach, an operator manually checks every fan after the blades of the fan have been riveted to a twisted spider. If one of the lobes is out of tolerance, the operator manually bends the lobe into a position where he believes the lobe and the attached blade are acceptable. The operator then re-checks the fan and repeats the bending process as necessary. Besides the expensive and time-consuming nature of manually bending lobes of an assembled fan, gripping and bending one of the lobes imparts torsional forces to the lobes not being bent by the operator. As such, the operator's bending of a single out-of-tolerance lobe to correct that lobe may cause one or more of the remaining lobes to be pulled out of tolerance.
Although the press machine generally imparts an identical twist angle to all of the lobes of a spider, variations in the spider material, alignment of the spider in the press machine, circumferential sliding of the lobes between the clamp pads, and spring-back of the spider lobes after twisting all contribute to twist angle variation between the lobes of the spider and on a spider-to-spider basis during a production run. These variations in twist angles cause variation in the balance of the fan after the fan blades have been connected to the lobes. An out-of-balance fan may operate inefficiently, may have a resonant frequency in an operating range of the fan, and may even cause an electric motor or other drive source to wear prematurely and even fail. To ensure that each fan produced using the traditional press machine is balanced, an operator uses a machine to check the balance of the fan about the rotation axis, manually clamps counterweights to the fan blades to correct for any imbalance, and then re-checks the balance of the fan. The process of adding counterweights and re-checking the balance of the fan is repeated until the balance of the fan is within acceptable tolerances. As will be appreciated, the process of manually balancing a fan using counterweights is an expensive and time-consuming process that adds significant cost to each fan produced. Further, industry standards typically limit fan suppliers to no more than five counterweights per fan. Thus, a fan that requires more than five counterweights to bring the fan within acceptable tolerances cannot be delivered to a customer and is scrapped.
Another approach to twisting fan spiders is disclosed in U.S. Pat. No. 2,611,414 to Sampatacos (“the '414 patent”), which utilizes jaws carried on cylindrical plungers to grip and twist lobes of a spider. The machine has an annular member engaged with the cylindrical plungers so that rotation of the annular member causes all of the plungers to turn in unison and to the same extent. Like the linkage of the press machine described above, the machine of the '414 patent uses an adjustable stop mechanism to limit movement of the annular member and control the twist angle imparted to the lobes of a spider. Although an operator may have an empirical initial setting for the adjustable stop, setting up the machine of the '414 patent to produce spiders having desired lobe twist angles would involve twisting a series of spiders and adjusting the stop after each twisted spider to bring the machine within tolerance. Like the traditional press machine, the series of twisted spiders produced during setup of the machine of the '414 patent would likely be scrapped which makes production runs of only a few spiders cost-prohibitive. Further, variations in the material properties of spiders throughout a production run would require constant adjustment of the stop of the '414 patent throughout a production run to limit lobe twist angle variation and the resulting balancing issues discussed above. The machine of the '414 patent is also similar to the press machine in that checking whether a spider is within tolerance and any necessary corrections would be done manually after the spider is removed from the machine. Further, neither the press machine nor the machine of the '414 patent compensates for spring-back of lobes of a spider after twisting, which can result in each of the lobes of the spider having different lobe twist angles even though they were all twisted to the same extent.